This is a randomized, multi-center, double-blind, single dose, placebo-controlled, dose escalating study of recombinant IL-12. Patients will be randomized to either IL-12 or placebo in a 4 to 1 ratio: Four cohorts of 10 patients will be entered. Patients must have a history of hyper-responsive airways to be eligible for this study. The primary objective of this study is to assess safety and tolerability of IL-12: a secondary objective is to assess the effects of IL-12 on the patients' baseline airway reactivity by evaluating pre- and post-dose methacholine PC20. This study is based upon the hypothesis that cytokines induced by T helper cells type 2 contributed to the physiological changes associated with asthma while cytokines produced by T helper cells type 1 antagonize these effects. IL-12 facilitates development of T helper type 1 cell differentiation and blocks the differentiation of T helper cell type 2. This is the first clinical study of IL-12 in patients with known airway hyperresponsiveness.